Selective circuits



Aug. 22, 1933.

P. R. COURSEY 1,923,354

SELECTIVE CIRCUITS Filed Feb. 12, 1931 I] U 4 I 7 l4 4 Hg 2 7 @9 -fla Fig.5 6

IHVENTOR FITTOEHEY Patented Au 2.22, 1933 I SELECTIVE CIRCUITS Philip litay Coursey, Richmond, England, assigner to Radio Patents Corporation, New orla, N. 51., a Corporation of New York Application February 12, 1931, Serial No. 515,269,

and in Great Britain February i l, 1930 I 9 Claims.

This invention has reference to circuits for selectively transmitting alternating electric cur rents, such as high or medium frequency oscillations. I More particularly, the invention applies to thermionic valve circuits and their use for the amplification of variable or alternating currents and voltages has for one of its main objects the provision an improved means for utilizing thermionic valves for such purpose.

A main object of my invention is the provision of means in a circuit or circuit system which serves for passing an alternating current energy of a definite frequency from one circuit to another, such as from the output circuit of an amoliiying valve onto the input circuit of a succeeding valve, whereby a considerable increase in the selectivity is obtained, without objectionable distortion of the signals, such as speech or music being transmitted on the alternating currents serving as a carrier.

Another object of my invention is to provide a selective circuit which, although it'is extremely selective to one particular frequency, possesses sufiicient uniform persistence as to the Whole range of notes comprised by a signal being transmitted by the alternating current as a carrier having its amplitude modulated in accordance with the signalling currents.

These and further objects of the invention will become more apparent by the following detailed description, taken with reference to the accompanying drawing,where I have illustrated, by way of example, a circuit diagram containing the invention.

Figure 1 shows a diagram of a valve amplifier embodying the invention, the partshaving no relation to the novel principle having been omitted.

Figure 2 is a similar diagram to Figure 1, illustrating an alternative method of practicing the invention.

Figure 3 shows an alternative form of the cou pling means in accordance with the invention.

Similar reference numerals identify similar parts in all the views of the drawing.

The invention in its particular application to thermionic amplifiers consists in the combination of thermionic valves of known types with inc-- proved coupling means for handing on the amplified energy, voltage and/or current from one valve to the next in the amplifier, whereby improved operating properties and characteristics are obtained. In the amplifying apparatus as heretofore constructed, it is known that in order to obtain the maximum util'zation of the apparatus, the coupling means between the amplifying valve stages should utilizea resonance action, whereby the amplifier is tuned to a particular frequency or band of frequencies. Such coupling means also the amplifying apparatus to be employed as a means of iiierentiating or sec-7 lecting between currents of different frequencies serving as carriers for diiierent signals. With such an arrangement the electrical constants of the resonant coupl" means determines the degree of selectivity obtainable with such an amplis her and a moderately high de ree of selectivity is only possible by the use of resonant circuits having a very low resistance or low energy losses and a consequent high degree of persistence. While such coupling circuits are satisfactory purely as a selective near for a particular frequency of constant a1np.. tude, they have great disadvantages when the current of voltage to be selected and amplified is varying in amplitude or as is, for instance, the casein the usual amplitude modulating systems used for communication and broadcasting purposes. By the term, .persiso once is understood the property of a circuit to maintain an electric current flow after the application of 'a'short impulse over a certain period of time after and beyond the duration or the im-' pulse. Assuming, for instance, a circuit comprised by a capacity and an inductance with a fairly high damping coefficient and a short cur.- rent impulse being applied to such circuit, the current will first rise to a maximum value determined by the impedance of the circuit, will persist for a short instant and then gradually die down again to zero. Substituting such a circuit by a circuit of medium damping, it is obvious that the current will rise to a higher value and then persist for a longer time, until it, in turn, begins to gradually, decrease to its zerovalue. Again assuming the same circuit with a fairly low damping, the persistence of the current will, in turn, be much greater and would become indefinite in case of a circuit with a zero damping 00- efiicient; that is, in the latter case a short. impulse of even infinitesimal duration would produce a continuous current flow over an indefinite period; This latter is an extreme and ideal case, which never happens in practice, but which serves to illustrate'the phenomenon of increased current persistence with decreasing damping coefiicient in the event the same exciting current impulse is applied to the. circuit in each case. Thus, itis readily understood. that the impulse which is 11 longest in the circuitwill have the best chance of notes and far too little high notes.

persisting in the built-up process or, in other words, the effects of persistence will be most marked in the case of low notes or frequencies. From this it follows that when using a highly efficient circuit with a minimum of damping to tune a valve, the output of this valve will appear to contain far too much of the low or bass The higher notes have actually come through, but they are drowned by those. lower down on the scale, which have a relatively higher persistence.

The high persistence of the resonant circuits, therefore, prevents the rapid response to the changes of varying amplitude of a modulated current, with the result that the amplified output from the apparatus will be distorted; that is to say, the changes will not be followed accurately but will be blurred.

According to the main object of the present invention, means are provided to overcome this defect by limitation of the. degree of persistence of the circuits, preferably to a low or moderate value, so that their response to changes in amplitude will be reasonably rapid and combining with certain of such circuits a means of destroying or damping their resonant properties at a specific any other kind of suitable damping means which is responsive to a single sharply defined frequency valve amplifier of moderate or high 7 frequencies, which are .of varying or modulated amplitude, two resonant or tuned circuits may be employed'as part of the coupling means, one being arranged in series with the anode circuit of one valve and the second joined in series with the coupling connection between said valve and the next following valve in the amplifier, so that principally it functions as a rejector for currents of the frequency to which it is tuned. With the successive amplifying stages of the apparatus arranged in suitable screen enclosures, the second above mentioned tuned circuit will prevent the handing on of any impulse to the second valve. The above mentioned absorption means, such as for instance a piezo electric crystal is connected across this tuned rejector coupling circuit, so that at this unique frequency the rejector action of the resonant coupling circuit will be destroyed and impulses of that frequency will be handed on to the second amplifying valve for further amplification. Thus, the selectivity of the coupling circuit connecting the amplifying tubes will, by this means, be much increased and its response limited substantially to a single frequency. Further, as this high degree of selectivity is achieved without the use of very low loss circuits, the amplifier will respond to rapid changes of current amplitude of the correct alternating or carrier current, without introducing distortion from the persistence of thecircuit.

Referring more particularly to the drawing, in-

which Figure 1 diagrammatically indicates one way of arranging the amplifier, separate screenspectively. The first of these-1 contains the radio receiver or other valve amplifier through which the incoming signal is obtained, whether from radio or any other source. This has not been illustrated in detail, as it is irrelevant to the subject matter of the invention, only the last valve having been indicatedschematically. The second screen compartment 2 contains the out put amplifier and other equipment, such as the rectifying means and others. The third compartment 3 contains the coupling means proper arranged between these two compartments. In compartment 1 an amplifying valve 4 is shown, which may be of any well known type with inductive and capacitative impedances 5 and 6 respectively in its anode circuit or coupled thereto in the usual manner, so that the amplifier or this stage of the amplifier is tuned to a definite and predetermined frequency or band of frequencies. .In the second screen compartment a second amplifying valve '7 is shown, which again may be of any well known type and which may, for instance, be the rectifying valve provided for this purpose, with the usual grid stopping condenser 8 and the grid leak 9 and associated equipment of well known type, which has not been shown for the sake of clearness of illustration and as having no relation to the particular coupling device of the invention enclosed in the compartment Eand serving as a means of coupling the two amplifier stages for selectively and without distortion handing on energy from the amplifying stagein l to the succeeding stage in 2.

In the compartment 3 a tuned rejector circuit is shown comprising inductance 10 and capacity 11, this rejsctor circuit being tuned to substantially the same frequency or band of frequencies for which the input amplifier contained in coinpartment' l is also tuned. The function of this rejector is, therefore, to prevent the passing on to the second part of the amplifier in 2 of any of the frequencies for which the amplifier in l is tuned. Across the tuned rejector circuit 10 and 11a selective damping device is shunted, in the case illustrated a piezo electric crystal 12, the purpose of which, as hereinbefore mentioned, is to damp out and destroy the rejector action of vthe circuit 10, 11 over a very narrow band of frequencies comprised within the wider band of frequencies to which the rejector circuit is tuned.

'very narrow bandof frequencies are handed on to the output circuit in 2, whereas the effect of unequal persistence of the high and low variations of the modulated current are completely or to a practically sufficient extent compensated on account of the relatively high electrical damping of the crystal, so that a substantially uniform response to high as well as low notes or variations is obtained. I This function is, furthermore, inrproved in the specific circuit embodiment due to the difference of the selective or resonant properties of the rejector circuit on the one hand and the piezo electric crystal on the other hand, the formerbeing purely electrical in nature and the latter purely mechanical, so that a mutual inter- .At this narrow orrejector band of frequencies,

ference of the resonant phenomena is thereby prevented.

In its broader aspects the effect of the device, as just described with reference to Figure 1, may be stated as being due, on the one hand, to the provision of electrical means for decreasing the persistence of the coupling circuit, thus insuring undistorted transmission of the current variations and, on the other hand, to theintroduction of an electro-mechanical oscillating device exhibiting the required high selectivity.

Other modifications and arrangements of the amplifying equipment can be devised in accordance with the terms of the specification and the broader aspects of the invention and the above is intended to be illustrative only of one of its methcdsf Thus, for instance, a further modification consists in the shunting of the resonant circuit first above mentioned and connected in series with the, anode circuit of the first amplifying valve by two selective energy absorbing means 13 and 14 (see Figure 2), such as piezo electric crystals having natural or absorbing frequencies slightly differing from one another. The fre quencies of cut-off of the amplification due to the resonant or tuned anode circuit can, therefore, be more precisely defined. The passing frequency of the damped rejector coupling circuit with the second amplifying valve is preferably given a value intermediate between these two outoff frequencies. With this arrangement the intervalve coupling means enclosed within the screen 3 is similarly arranged and functions in the same manner as described in connection with Figure 1.

A further modification consists in the substitution of a partially tuned choke coil, or an aperiodic circuit or resistance for the resonant circuit connected in the anode circuit of the first amplifying valve. This alternative anode circuit impedance may be used either with the simple intervalve coupling means-Figure 1, or in association with two shunting damping means 13, 14, as in Figure 2.

Additional selectivity by band-pass filter and other similar complex resonant circuits may likewise be obtained by adding these in any wellknown manner to the amplifying arrangement forming the subject of this invention.

This combination of damping and coupling means may likewise be applied to such complex circuits, as well as tothe simple resonant circuits described above.

In Figure 3 the piezo electric properties of the quartz crystal 15 are utilized in a modified manner, to provide the coupling means between the amplifying stages. The crystal is, provided with two pairs of electrodes or contacts 16, 17 and 18, 19, which are preferably arranged so that the electrostatic capacity between them is small. The sole coupling means between the circuits connected to 16, 17 and 13, 19 is thus provided by the mechanical reactions of the piezoelectrically sensitive material. The application of variable electrical potentials to the first mentioned electrodes will result in mechanical vibration of the said piezo electric material, provided that the frequency of the variations of potential are very nearly or quite identical with the natural or resonance frequency of the material. The mechanical vibrations set up in manner will excite corresponding electrical disturbances in the second or output electrodes, which will influence the second stage of the amplifier. The material thus acts as a coupling responsive substantially only'to a single frequency, since the said second amplifying stage will receive no impulses of other frequencies. The electrode pairs 16,17 and 18, 19 are preferably arranged at an angle, such as right angle, in respect to each other and the crystal body, as shown, but any other arrangement, such as a parallel arrangementeach pair at one end of the crystal plate will give satisfactory results. To aid in the operation of the amplifier, the input and output stages on each side of the coupling means should preferably be adequately screened from one another.

As, a further aid'to increase the selectivity provided by this coupling means, the input stage 4 ofthe amplifier may have the impedance in its anode circuit, comprising the tuned circuit 5, 6, a tuned or semi-tuned choke, or a resistance, etc. shunted by a pair of piezo electric or .similar damping means, such as 13, 14 in Figure 2. By this modification the band of frequencies to which the amplifier is responsive may be further narrowed down. I

A plurality of such coupling means may be incorporated in a single amplifying apparatus-arranged between the same pair of or between different stages of the amplifier. The coupling means arranged in accordance with this invention in conjunction with a valve amplifying equipment may utilize any substance exhibiting piezoelectric properties, or may employ magnetostrictive or other similar properties, provided that appropriate input, and output electrodes or other coupling means to the material are adopted.

What I claim is:- v 1. A selective circuit for passing modulated high frequencyv currents from an input to an output circuit, connected therewith comprising a rejector circuit individual to and serially arranged in said circuit and broadly tuned to the frequency of said currents to normally prevent the passage of said currents and an electro-mechanical frequency selective device associated with said rejector circuit to provide a'sharply tuned coupling path for said high frequency currents.

2. A selective circuit for passing high frequency currents, an input circuit, anoutput circuit, connected therewith, a rejector circuit individual to and serially arranged with regard to said circuit and consisting of capacity'and inductance in,

parallel relationship tuned to the frequency of said currents and arranged to normally prevent the passage of said currents from saidinput to said output circuit and an electro-mechanical frequency selective'device connected in parallel I to said rejector circuit to provide a sharply tuned coupling path for said high frequency currents.

-3. A selective circuit arrangement, comprising an input circuit, an output circuit, an intermediate circuit between said input and said output circuit for handing on high frequency currents from said input to said output circuit, said intermediate circuit including a rejector circuit serially arranged therein and tuned to the frequency of said currents to normally prevent their passage from said input to said output circuit and a piezo electric crystal device connected inparallel relationship to said rejector circuitand having a natural oscillating frequency equal to the fre- 'quency of said currents.

4. In a selective circuit arrangement for transmitting modulated high frequency currents, com-' quency currents from said inputto said output circuit, said intermediatecircuit including a serially arranged rejector'circuit tuned to the frequency of said currents to normally preventtheir passage through said intermediate circuit and a piezo electric crystal device having .a natural oscillating frequency equal to the frequency of said currents placed in parallel relationship to said rejector circuit for providing a highlyselective coupling path of said high frequency currents. I r

5. In a selective circuit for transmitting modulated high frequency currents; input and output coupling means for applying input currents to and withdrawing output currents from said circuit; a highly damped rejector circuit serially inserted in said circuit tuned to the frequency of the currents to be transmitted; and an electromechanical frequency selective device connected in parallel to said rejector circuit.

6. In a selective circuit for transmitting high quency response characteristic including the frequency of the currents to be transmitted by said circuit; and a-piezo electrical crystal device tuned to the frequency of the currents to be transmitted and connected in parallel to said rejector circuit.

7. In a selective circuit for transmitting high frequency modulated currents; input and output coupling means for applying input currents to and withdrawing output currents from said circuit; a highly damped rejector circuit consisting of inductance and capacity in parallelserially inserted in said circuit and having a frequency response characteristic including the frequency of the currents to be transmitted by said circuit; and a pair of piezo electric crystals connected in parallel to said rejector circuit and having different fundamental natural frequencies to determine the cut-off limits of the frequencies built up in said circuit.

8. In a high frequency system, a first circuit; a second circuit receiving high frequency currents from said first circuit; a rejector circuit consisting of an inductance and a condenser in parallel'being serially inserted in and individual to said first circuit, said rejector circuit being highly damped and tuned to a desired transmission frequency; a pair of piezo electric crystal devices shunting said rejector circuit and having different fundamental frequencies to determine the cut-off limits of the frequencies built up in said rejector circuit; a second rejector circuit similar to said first rejector circuit serially inserted in and individualto said second circuit and also tuned to said desired transmission frequency anda further piezo electric crystal device shunting said second rejector circuit and having afundamental frequency midway between the fundamental frequencies of said pair of said piezo electric crystals.

9, In a radio receiving circuit comprising a first receiving tube; a second receiving tube to receive high frequency energy from said first tube; a rejector circuit consisting of an inductance and a condenser in parallel and serially inserted in the anode circuit of said first tube; a pair of piezo electric crystal devices shunting said rejector circuit and having different fundamental frequencies to determine the cut-01f limits of the-frequencies built up in said rejector circuit and applied to the grid of said second tube; a second rejector circuit similar to said first rejector circuit and being serially inserted in the grid circuit-of said second tube and tuned also to said desired transmission frequency and a further piezo; electric crystal device shunting said second rejector circuit and having a fundamental frequency midway between the fundamental frequencies of said pair of piezo electric crystals.

' PHILIP RAY COURSEY. 

